This invention relates to a mobile station for use in a communication system and, in particular, to a method of controlling transmission power in the mobile station.
Heretofore, considerable attention has been devoted to a CDMA mobile radio communication system as a communication system of the type described. In such a CDMA mobile radio communication system, a plurality of base stations are spatially dispersed or arranged to define service areas, respectively, like in any other mobile radio communication systems. However, the CDMA mobile radio communication system is advantageous in that the same frequency can be used in each of the base stations to transmit a sequence of digital signals and a frequency resource can be therefore effectively utilized therein. In other words, radio channels are defined by a common frequency.
In this event, each digital signal is transmitted in the form of spread spectrum signals by using a code peculiar or pre-assigned to each of the base stations. In addition, a pilot signal is also usually pre-assigned to each base station to specify the respective base stations and is generated at a constant power level from each base station. Therefore, such a pilot signal may be called a constant power signal which is generated at a predetermined power level from each base station.
Herein, it should be considered that each of mobile stations is movable through various service areas of the base stations and must be therefore communicable with every base station. Thus, each mobile station is moved or traveled through the plurality of the service areas. In this event, each of the mobile stations continues to carry out communication during movement without any interruption of communication, switching the base stations from one to another. Such a switching operation will be referred as a handoff or hand-over operation.
On the other hand, each of the service areas is not always geometrically completely separated from one another but has a boundary spatially and partially overlapped with one another. Taking this into consideration, each mobile station is put into a state which is communicable with a plurality of the base stations when the hand-off operation is carried out within an overlapped area. In other words, the hand-off operation is carried out by the use of transmission and reception between a single mobile station and a plurality of the base stations in the CDMA cellular mobile radio communication system. Such a band-off operation is generally called a soft hand-off operation.
Herein, it is to be noted in the CDMA cellular mobile radio communication system that transmission electric power must be always minimized at each of the mobile stations while communication is usually carried out through a radio channel with the base station. Otherwise, interference inevitably takes place in the other radio channels because the same frequency is used in common to the other base stations. Consequently, the interference for the other radio channels can be reduced to a minimum. This enables concurrent communication of each base station with a lot of the mobile stations.
Taking the above into consideration, transmission power control should be executed in each mobile station to measure a transmission loss of each base station and to select the base station of a minimum transmission loss. As a result of the transmission power control, each mobile station is always connected to the base station of the minimum transmission loss.
More specifically, such transmission power control which is executed in each mobile station is classified into first and second types of transmission power control. In the first type of transmission power control, an open loop type of control operation is carried out by measuring a reception level of each pilot signals sent from the base stations and by determining the transmission power in each mobile station with reference to the results of measurements. In the second type of transmission power control, a closed loop type of control operation is made by measuring a quality of communication or a reception level of a desired signal in each base station under connection and by indicating transmission output power to the mobile station so that the quality of communication or the reception level is kept at a constant.
The first type of transmission power control can quickly control the transmission power in each mobile station on the basis of the reception level of the pilot signals measured at each mobile station and enables high speed control operation in response to an abrupt variation of the transmission loss. On the other hand, the second type of transmission power control responds to a variation of a desired signal due to fading and enables precise control operation so as to guarantee a constant quality of communication or a constant reception level of the desired signal. In other words, the second type of transmission power control is slow in control speed in comparison with the first type of transmission power control.
Under the circumstances, consideration is made about a mobile station which adopts such two types of transmission power control and which communicates with a certain base station under connection, which may be called an old base station. In addition, it is assumed that the mobile station is suddenly switched to a new base station under non-connection by the soft hand-off operation. In this event, it is preferable that the transmission power between the mobile station and the base station under connection is kept at a low level until connection is completed between the mobile station and the new base station during the soft hand-off operation in order to avoid the interference for the any other mobile stations.
However, it has been found out according to the inventors"" experimental studies that the transmission power between the mobile station and the base station under connection, namely, the old base station practically becomes excessively large during the soft hand-off operation and, as a result, occurrence of interference among the radio channels is inescapable due to such excessive transmission power of a single mobile station.
It is an object of this invention to provide a transmission power control method which is used in a mobile station and which can prevent interference during a hand-off operation.
It is another object of this invention to provide a method of the type described, which can reduce an excessive increase of transmission power in the mobile station during the hand-off operation.
It is yet another object of this invention to provide a method of the type described, which is applicable to the mobile station in a CDMA cellular radio communication system.
It is still another object of this invention to provide a mobile station which is capable of reducing an increase of transmission power during a hand-off operation.
A method to which this invention is applicable is for use in controlling transmission power in a mobile station which is used in a CDMA radio communication system. The CDMA radio communication system comprises a plurality of base stations each of which transmits a pilot signal pre-assigned to each base station and a control signal for power control of the mobile station. The mobile station successively controls transmission power in response to the control signal sent from each of the base stations during communication with the base stations. According to an aspect of this invention, the method comprises the steps of periodically detecting, in the mobile station, a level of each pilot signal sent from the plurality of the base stations, checking out whether or not the mobile station is being connected to the plurality of the base stations, discriminating between a first group of the base stations under connection and a second group of the base stations under non-connection, comparing a first reception level of the pilot signals concerned with the first group of the base stations with a second reception level of the pilot signals concerned with the second group of the base stations to determine whether or not the first reception level is higher than the second reception level, and suppressing an increase of the transmission power in the mobile station by reducing the transmission power when the second reception level exceeds the first reception level.
When each of the mobile stations comprises first transmission power control means for adjusting the transmission power on the basis of a reception level concerned with the pilot signals and second transmission power control means for controlling the transmission power by a control step size on the basis of a control instruction sent from the base stations under connection, the method according to another aspect of this invention comprises the steps of receiving the pilot signals from the plurality of the base stations, determining the transmission power in inverse proportion to a maximum one of reception levels of the pilot signals, judging whether or not a first reception level of the pilot signals concerned with the base stations under connection exceeds a second reception level of the pilot signals concerned with the base stations under non-connection to produce a result of judgment, changing the control step size from one to another when the second reception level exceeds and does not exceed the first reception level.
The changing step may comprise the steps of decreasing the control step size when the second reception level exceeds the first reception level and increasing the control step size when the second reception level does not exceed the first reception level.
Alternatively, the changing step comprises the steps of rendering the control step size into a small step size for an increase of the transmission power when the second reception level exceeds the first reception level and rendering the control step size into a large step size for a decrease of the transmission power when the second reception level does not exceed the first reception level.
According to a further aspect of this invention, the method comprises the steps of receiving the pilot signals from the plurality of the base stations; determining the transmission power in inverse proportion to a maximum one of reception levels of the pilot signals, judging whether or not a first reception level of the pilot signals concerned with the base stations under connection exceed a second reception level of the pilot signals concerned with the base stations under non-connection to produce a result of judgment, minimizing the transmission power at every time interval for a predetermined time period when the second reception level exceeds the first reception level, and executing a control operation by the use of the first and the second transmission power control means either during a time period except the predetermined period or when the second reception level does not exceed the first reception level.
According to yet another aspect of this invention, the method comprises the steps of receiving the pilot signals from the plurality of the base stations, determining the transmission power in inverse proportion to a maximum one of reception levels of the pilot signals, judging whether or not a first reception level of the pilot signals concerned with the base stations under connection exceeds a second reception level of the pilot signals concerned with the base stations under non-connection to produce a result of judgment, decreasing the transmission power in the mobile station by a predetermined amount at every preselected time interval when the second reception level of the pilot signals exceeds the first reception level, and executing a control operation by the use of the first and the second transmission power control means either during the preselected time interval or when the second reception level does not exceed the first reception level.
Herein, the transmission power in the first transmission power control means may be determined in inverse proportion to a reception level concerned with a whole of the pilot signals.
According to still another aspect of this invention, the method comprises the steps of periodically detecting each pilot signal level from a plurality of base stations, checking out whether or not the base stations are under connection to discriminate between a first group of the base stations under connection and a second group of the base stations under non-connection, and reducing the transmission power in the mobile station to suppress an increase of the transmission power in the mobile station when a first reception level concerned with the first group of the base stations does not exceed a second reception level concerned with the second group of the base stations.
A method to which this invention is also applicable is for use in controlling transmission power in a mobile station in a radio communication system which comprise a plurality of base stations. The mobile station is continuously communicable through radio channels with the base stations by executing a hand-off operation during a hand-off time. According to another aspect of this invention, the method comprises the steps of detecting the hand-off time and lowering the transmission power in the mobile station during the hand-off time to mitigate interference among the radio channels.
A mobile station to which this invention is applicable is for use in a communication system which comprise a plurality of base stations. The mobile station is continuously communicable through radio channels with the base stations by executing a hand-off operation during a hand-off time. According to this invention, the mobile station comprises means for detecting the hand-off time and means for lowering the transmission power in the mobile station during the hand-off time to mitigate interference among the radio channels.